A commercial method for manufacturing titanium dioxide pigment, known as the chloride process, is based on titanium tetrachloride (TiCl4) being converted into titanium dioxide and chlorine gas in a tubular reactor using a preheated, oxidizing gas, such as oxygen, air, etc., and certain additives. The oxidation reaction is highly exothermic, meaning that the reaction mixture displays temperatures of more than 1,500° C. following complete conversion. In a downstream reactor cooling section, the TiO2 pigment particles formed are cooled to below roughly 400° C. and separated from the gas flow. Cooling directly after the completion of particle formation must take place rapidly in order to prevent further particle growth. To this end, the tubular reactor or the reactor cooling section is externally cooled with water from this point onwards.
However, the transfer of heat to the cooling water is severely impeded by the accumulation of TiO2 pigment particles on the inner wall of the tubular reactor or the reactor cooling section. According to U.S. Pat. No. 2,721,626, scrub solids are introduced into the reactor cooling section for detaching pigment particles that have accumulated on the inner walls. The scrub solids used in U.S. Pat. No. 2,721,626 are abrasive particles, such as quartz sand or aggregated TiO2 particles with particle sizes of roughly 0.15 mm to 6.35 mm. The scrub solids are introduced into the TiO2/gas suspension at one or more points in the reactor cooling section.
Because of their weight, the scrub solids begin to concentrate in the lower one-third of the reactor tube circumference in the horizontal reactor cooling section just a short time after being added. While this area of the inner wall is thoroughly cleaned of adhering pigment, the higher areas of the circumference are insufficiently cleaned, and the cooling of the gas suspension is inadequate. Nevertheless to achieve sufficient heat transfer, it is standard practice to substantially increase the amount of scrub solids added. The added solids increase the burden on the system for manufacturing, adding and eliminating the scrub solids, thus giving rise to higher costs for energy consumption and maintenance, among other things.
U.S. Pat. No. 6,419,893 B1 describes a method for more efficient removal of the TiO2 deposits on the inner wall of the reactor cooling section. According to U.S. Pat. No. 6,419,893 B1, at least a partial area of the reactor cooling section is provided with ribs that run in helical fashion on the inner wall and serve as guide elements, as a result of which the scrub solids are guided through the cooling section in a helical flow. The ribs are arranged at an angle of 2° to 6°.
US 2006/0133989 A1 discloses a reactor cooling section of helical overall design that is said to achieve improved cleaning of the inner wall by the scrub solids.
U.S. Pat. No. 3,735,000 discloses a method for manufacturing titanium dioxide by reaction in the gas phase, where part of the gaseous reaction components is introduced tangentially into the reactor. This method is designed, to reduce the formation of deposits on the reactor walls by tangential introduction of one reaction component while achieving rapid thorough mixing of the reaction components by generating a back flow. The back flow is further intensified by the cross-section of the reactor expanding conically in the direction of flow. However, back flow leads to residence times of different length for the individual particles in the reactor.
A narrow particle size distribution is important for the quality of a titanium dioxide pigment, particularly for the tinting strength (TS). However, for generating a narrow particle size distribution, not rapid thorough mixing of the reaction components is of primary importance, but a narrow residence time distribution of the TiO2 particles in the reactor, meaning that any kind of back flow in the reactor should be avoided.